Modern humans are heavier than our hunter-gatherer ancestors, but our bones are lighter. Now, an international team of researchers knows why: farming.

As human society developed agriculture and shifted to a more sedentary lifestyle, beginning some 12,000 years ago, people no longer had to spend most of their days walking, running and lifting to get food.

Those activities put bones under stress, and cause them to grow stronger.

The researchers analyzed arm and leg bone samples from hundreds of humans who lived during the past 33,000 years in Europe. They found a gradual weakening as communities became agricultural, and that moving into cities, changing diets or other lifestyle changes had little impact.

Taking molds of bones from museum collections and comparing femurs to arm bones - which would not be affected by the amount of walking or running, they were able to determine that the changes were due to a decline in mobility.

Writing in Proceedings of the National Academy of Sciences, the researchers said leg bone strength declined between the Mesolithic era, 10,000 years ago, and the time of the Roman Empire, which began about 2,500 years ago. Arm bone strength remained the same.

Our lighter bones make us more susceptible to osteoporosis, at higher risk of broken bones. But the researchers note that weight-bearing exercise can prevent bone loss, and could help people develop more a Paleolithic-style skeleton.